Methods: :
Adult wheel-running mice were trained to associate the presentationof a light flash with the brief availability of water at a spoutin their cage. The light flash stemming from an intensity-controlledLED and mounted above the wheel was randomly triggered as themouse ran on the wheel. A mouse was deemed to have detectedthe flash if it immediately ceased running within one revolutionafter the flash. A brief, 5 deg. test flash ( = 500 nm or 365nm) was delivered in darkness or in the presence of a whiteGanzfeld background light to the ventral retina. ‘Frequencyof seeing’ data were collected with the method of constantstimuli. (1 R*s-1 stands for 1 photoisomerization per rod persecond).

Results: :
(1) In both WT and Gnat-2-/- mice, the flash produced 5-20 photoisomerizationsat absolute threshold. (2) On a background that produced about0.008 R*s-1, the rod threshold was raised to twice the absolutethreshold. (3) In WT mice, a 365 nm test flash presented ona moderate background [25 R*s-1] was mediated by cones containingUV-opsin; in Gnat-2-/- mice, the 365 nm flash was mediated byrods. (4) In WT mice, at higher background levels [300 - 400R*s-1], the 500 nm test flash was mediated by cones that containM-opsin; in Gnat-2-/- mice, the 500 nm flash was mediated byrods. On backgrounds that produce ~30,000 R* s-1, the rods ofGnat-2-/- mice did not yet show saturation.

Conclusions: :
Rod vision of Gnat-2-/- mice is very similar to that of WT micein low and moderate adapting ranges. Under very bright light,the sensitivity of the rods in Gnat-2-/- mice may be regulatedby a light-adaptive mechanism that involves the translocationof transducin, from the rod photoreceptor outer segment to theinner segment as described by Sokolov et al. (Neuron, 2002).